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This UMA will report whether CDM instance creation succeeded or not. As
usually, we report it for Clear Key, Widevine and other (unknown) key
systems.

BUG=700147
TEST=Manually checked about://histograms

Change-Id: I0925d07c7a61da68ba43d2d170931617a4452f07
Reviewed-on: https://chromium-review.googlesource.com/727180Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510258}